1. Field of the Invention
The invention relates to drilling or coring installations, in particular for drilling oil wells.
The invention relates more particularly to a device intended to be associated with a drilling or coring installation, in order to monitor it, by means of measurements of well parameters.
2. State of the Art
Drilling and coring of wells are normally carried out by means of a drill bit which is connected to a drive motor, located on the surface, by means of a drill string. Progressively as the drill bit advances in the well, pipes are added to the drill string.
It is desirable to have available a maximum amount of data relating to the drilling or coring operations, particularly regarding the medium in which drilling is taking place and the behaviour of the drill bit and its cutting head. Data relating to the medium in which drilling is taking place include in particular the type of rock formation attacked, the composition of the drilling mud and the presence of oil or other fluids. Data relating to the drill bit and its cutting head include its instantaneous rotational velocity, the variations in rotational velocity, the position of the cutting head relative to the wall of the well being drilled, the variations in the rotational velocity and in the velocity of advance in the well, the lateral and axial impacts to which the drill bit is subjected and the precession motion of the drill bit (generally called “whirling” in the English-language literature).
These data or drilling parameters can be stored during the drilling or coring operation and used subsequently to analyse the problems which may have arisen during the drilling or coring operation (such as, for example, momentary and unexpected slowing of the penetration velocity of the cutting head in the rock formation or abnormally rapid wear of the cutting head) or to adapt the conditions of other drilling or coring operations.
To detect the data or parameters mentioned above, drilling installations are provided with suitable measurement equipment which is arranged in the drill string or in the drill head.
Thus, in document BE-1007274, a drill bit is described the cutting head of which contains judiciously distributed accelerometers to determine the vibrations to which it is subjected during a drilling operation. In this known drill bit, the accelerometers are positioned in the drill head, which makes the acquisition of the parameters only possible for this particular drill head thus equipped.
In document U.S. Pat. No. 4,303,994, a drilling installation is described including a drill bit, a drill string and measurement gauges, which are housed in the drill string. In this document, the measurement gauges are arranged in the upper part of the drill string but information is not provided as to the manner in which they are inserted in the drill string. The arrangement of the measurement gauges in the upper part of the drill string constitutes a disadvantage, as the measurements which they perform do not take into account distortions suffered locally by the lower part of the drill string between the measurement gauges and the drill bit. These distortions include in particular flexions and torsions the characteristics of which vary over time, during the advance of the drilling. There results a sometimes considerable discrepancy between the measurements read by the gauges and the true operating characteristics of the drill bit and of its cutting head. In addition the assembly of the measurement gauges is relatively long, so that it is normally impossible to position it between the drill bit and a downhole motor.
Moreover, the known measurement equipment, described in the above-mentioned documents is not generally standard, but must be adapted, in each case, to the drill bit or to the drill string used, which complicates the construction of these devices, forms an obstacle to mass production and increases the cost thereof.
The drilling installations described in documents GB-2 344 127 and WO-2006 087 239 partially overcome the disadvantages of the devices of documents BE-1007274 and U.S. Pat. No. 4,303,994. In these known installations, the measurement equipment is arranged in one or more chambers, formed at the periphery of couplings which are incorporated in the drill string of the drilling installation. This arrangement permits easy access to the measurement elements. However it presents various disadvantages, which are stated below.
A first disadvantage lies in the introduction, into the drill string, of a coupling of low mechanical strength. The coupling is in fact subjected to forces which can be large during a drilling operation, in particular axial compression or torsion forces, or else flexion moments. By hollowing out the peripheral zone of the coupling to form the chamber intended for the measurement equipment there, its mechanical strength is inevitably reduced. The torsion, as well as the flexion moments in fact act mainly on the outer fibres of the coupling. As these fibres are interrupted by the presence of the peripheral chamber, the torsion and flexion forces are channelled in the central part of the coupling, which has a smaller diameter, which exposes it to premature breakage.
In the installations of documents GB-2 344 127 and WO-2006 087 239 the low strength of the coupling in flexion and in torsion has the disadvantageous consequence of interfering with the proper functioning of the measurement equipment, leading to untimely modification of the behaviour of the drill string.
In these known installations, the coupling is in fact provided with an outer sheath, intended to obturate the chamber containing the measurement equipment. This sheath can absorb part of the forces which the measurement equipment has precisely the task of measuring. As a result the calibration of the measurement equipment is rendered difficult.
The presence of the sheath at the periphery of the coupling is the cause of another disadvantage of these known drilling installations. This sheath is in fact exposed to the drilling mud rising back in the annular zone situated between the drill string and the wall of the well. This drilling mud rising back along the drill string is charged with rock cuttings which have been extracted by the action of the drilling tool. In addition, since this sheath is exposed to the drilling fluid in the outer part of the device, it is subjected to the hydrostatic pressure from the drilling fluid, which can be high in the case of very deep wells. Consequently the sheath must be very thick in order to withstand this hydrostatic pressure.
During drilling, the sheath enters into contact with the wall of the well. It can therefore be damaged, be corroded, wear, become unscrewed, or be punctured. This can therefore result in malfunctioning of the device. This can also result in a great deal of difficulty when the sheath is removed for maintenance of the measurement equipment.
The sheath poses another problem during handling of the coupling and, more generally, of the drill string. It is in particular subjected to large torsional forces which can damage it, when screwing the coupling onto the drill string. This is particularly the case when the sheath is designed to minimise the absorption of forces in order to reduce its effect on the taking of measurements.